The Correlated Color Temperature indicates the color of the light. The CCT is based on a Kelvin scale, which is actually a temperature measurement, but in this usage has no bearing on the actual temperature of the light or the heat it generates. The Kelvin scale is like the Celsius and Fahrenheit Scale, but starts at 0 to denote absolute zero – which is as cold as we think it can possibly get.

You may ask why a temperature scale is used to measure light color. Here’s why:

Many years ago it was noticed that when a piece of iron burns (called technically a “black body”), it changes color as it gets hotter, starting from an orange color, then yellow color and getting to a blue -white hot color. The temperature of iron burning is measured on the Kelvin scale. This observation was then used to describe the color of light.

Strangely though, the higher the “temperature”, the cooler the light is perceived and the lower the “temperature”, the warmer the light is perceived.

A CCT below 3200 K is considered a “warm” light, with more of a yellow tint and a higher CCT, above 4000 K, is considered a “cool” white, with more of a blue tint.

High Pressure Sodium – common highway light

1900 K

Warm Compact Fluorescent

2700 K

Incandescent

2700 K

Halogen Light - common spotlight

3200 K

Cool White Linear Fluorescent - typical office light

4200 K

When you purchase a light it is important to know the Kelvin color of the light. For a home environment, most people want a lower Kelvin, about 3000K or less. For an office environment, around 4500K is acceptable. Many bulb manufacturers now indicate the Kelvin color on the packaging of the bulb or on the bulb itself. This would be shown as “4000K” on the base of the bulb. Sometimes the packaging indicates simply that the bulb is a warm or a cool white.

The Department of Energy is pushing manufacturers to label their products with lumens and Kelvin color so consumers can accurately select the correct light for their application.

The color of light is also important in an outdoor application. A warm or yellow light is often used for landscape lighting to give a warm glow around a home. While many outdoor lights used for general illumination, such as street lighting or parking lot lighting are still quite warm due to the widespread use of High Pressure Sodium Lighting, there is a push to use higher Kelvin lighting such as Metal Halide lights as studies have shown that the eye can see better at night with this type of lighting.